1. Field of the Invention
The present invention relates to a non-linear amplifier and a non-linear emphasis/deemphasis circuit using the same.
2. Description of the Related Art
In general, a VTR (video tape recorder) or video disk records a video signal through FM modulation. In recording or reproducing and demodulating an FM modulated signal, the noise distribution after FM demodulation increases as the frequency increases. A so-called triangular noise will occur. As a method for suppressing this triangular noise to improve the S/N ratio, an emphasis/deemphasis technique is used. According to this technique, the high frequency component of a video signal is emphasized by an emphasizer, the resultant signal is subjected to FM modulation, and at the time of demodulation the high frequency component is suppressed by a deemphasizer having the opposite characteristic to the emphasis characteristic.
The emphasis characteristic is restricted by the transfer band of an FM waveform, and if the amount of emphasis is increased too much, a so-called inverting phenomenon would occur in which the white peak of the video signal falls down to the black level. As a method to avoid this inverting phenomenon and further improve the S/N ratio, a non-linear emphasis has been proposed. The non-linear emphasis reduces the amount of emphasis when the amplitude of a video signal is high and increases that amount when the amplitude is low. The non-linear emphasis is disclosed in, for example, "Signal Processing in Four-hour Recording VHS System VTR" in National Technical Report Vol. 25 No. 1 Feb. 1979. The non-linear emphasizer disclosed in this article can avoid the inverting phenomenon. To attain a non-linear emphasis characteristic to a region where an input signal has a minute level (e.g., -26 dB or above), however, the level of the input signal should be set sufficiently large in advance depending on an amplifier so that the non-linear region of a diode can be utilized even in the minute-level region. Consequently, when the intrinsic signal amplitude is high, a signal in the emphasizer would have an excess amplitude. This requires an increase in breakdown voltage of a diode as well as a source voltage.
For a non-linear deemphasizer, it is ideal that the gain A of an amplifier is infinite. The gain A however takes a limited value, and the group delay characteristic and frequency characteristic are deteriorated particularly for a high frequency. It is not therefore possible to attain a non-linear deemphasis characteristic having the totally opposite to the characteristic of a non-linear emphasizer. If the non-linear emphasis characteristic is not completely opposite to the non-linear deemphasis characteristic, a reproduced video signal after FM demodulation may have ringing thereon or have a deteriorated waveform characteristic.
As described above, according to a conventional non-linear emphasis/deemphasis circuit, if the emphasis characteristic should be realized even in a region where an input signal has a minute level, a signal in the circuit would have an excess amplitude. This requires an increase in breakdown voltage of a diode or source voltage. Further, deterioration of the group delay characteristic and frequency characteristic at a high frequency does not permit that an emphasizer and a deemphasizer have completely opposite characteristics, thus impairing the quality of a signal.